1. Field of the Invention
This invention relates to a method and apparatus for the surface area determination of solids that is free from operator bias and produces reliable results.
2. Discussion of the Prior Art
Various prior art methods have been used to determine the surface areas of solids. The most effective and commonly used is the method of S. Brunauer, P. H. Emmett and E. Teller described in the J.Am.Chem.Soc. 60,309 (1938). This method, commonly referred to as the BET method, involves the determination of the volume of adsorbate gas (usually nitrogen) adsorbed, then desorbed by a sample at relative pressures below 0.35 psi. The BET method was later improved by the U.S Bureau of Standards. (See W. V. Loebenstein and V. R. Dietz) J. Research Natl. Bur. Standards, vol. 46, No. 1, pp 51-53 (1951).
The invention of Fred M. Nelsen and Frank T. Eggertsen, U.S. Pat. No. 2,960,870, was the first apparatus and method that was faster and simpler than the old vacuum systems it replaced, but was still limited in acceptance because it was too slow to effectively monitor on-going processes in factories that were trying to measure and control the surface area of their products. The instrument required skilled operators who were familiar with gas chromatographic techniques. However, their instrument (the Shell Sorptometer) was the first embodiment of the BET method to result in a practical instrument that was accurate, and applicable over a wide range of surface areas. Most importantly, the Shell device provided an improved method of calibration which presented direct read-out of surface area without resorting to reference materials. (See Section 5, lines 36 to 43 and lines 51 and 52 of U.S. Pat. No. 2,960,870.)
In general, the Nelsen and Eggertsen "Shell Sorptometer" utilized gas chromatographic techniques, in what has become known as the dynamic flow system, to deliver the adsorbate nitrogen, at relative pressures below 0.35 psi. to the sample. The sample was then cooled in liquid nitrogen, and the volume of nitrogen adsorbed, then desorbed by the sample was determined by integration of the signals obtained from the thermal conductivity bridge. The Shell Sorptometer required the precise and frequent injection of known volumes of nitrogen (by syringe) in order to keep the unit in calibration. The detectors were run at elevated temperatures, and were very sensitive to changing ambient conditions. As a result, it was essential to constantly balance, or null the wheatstone bridge detecting circuits comprised by these thermal conductivity detectors.
All dynamic flow instruments are subject to the variations in null point caused by temperature variations of the flowing gas stream at the onset of the adsorption cycle when the sample, and the gas mixture flowing through it are immersed in liquid nitrogen. These fluctuations produce "phantom" signals which are so similar to "true" adsorption signals that they will be misread by the logic and command circuits of an automatic surface area analyzer which will then respond in an inappropriate manner; for example, the "phantom" signal will be thought to be the adsorption signal, and when it (the "phantom" ends, and the detector returns to null, the logic circuitry will read that as the completion of the adsorption cycle, which has not yet begun, and lower the bath of liquid nitrogen, to permit the desorption process to begin.
To counter these fluctuations in the detector caused by these temperature changes, efforts were made to make the detector immune to these temperature changes. The detectors were immersed in a constant temperature bath (see U.S. Pat. No. 2,960,870 lines 4-18 to 4-21), or more recently in proportionally controlled ovens such as used in the Flowsorb 2300 (manufactured by Micromeritics Corporation, Norcross, Ga. These attempts to control the temperature of the detector to avoid the drift and fluctuations caused by the shifting temperature of the gas mixture, compromised the reliability and serviceability of these instruments and still required the operator to readjust two bridge balance controls throughout the entire analysis.